Low noise amplifiers used in modern communications systems such as mobile phones often need to be driven in balanced or differential mode. In other words, they need to be fed by two input potentials with the same amplitude and with a 180 degrees phase difference. Usually, to protect the receiver against interferences, a filter is placed after the antenna. It is very common that the same antenna is used for both the transmitter and the receiver. In this case, depending on the communication system, the separation between the transmit path and the receive path is done with a switch (when they occur at different times) or with a duplexer that permits a frequency separation of a signal, as described by way of example with reference to U.S. Pat. No. 7,196,594 to Cheema et al. for a SAW Duplexer Having Enhanced Isolation Performance, the disclosure of which is herein incorporated by reference in its entirety. In both cases, it is advantageous to have a receive filter or a duplexer with a balanced output signal.
As is well known in the art, one very efficient technology useful in making RF filters or duplexers includes surface acoustic wave (SAW) devices. SAW ladder filters and resonator filters are two common design approaches to realize SAW RF filters and duplexers, as described by way of example with reference to U.S. Pat. No. 6,833,774 to Abbott et al. for a SAW Filter and U.S. Pat. No. 7,105,980 to Abbott et al. for a SAW Filter Device and Method Employing Normal Temperature Bonding for Producing Desirable Filter Production and Performance Characteristics, the disclosures of which are herein incorporated by reference in their entirety. The SAW ladder filter typically comprises series and shunt resonators electrically cascaded in a ladder network to form the filter. Each resonator consists of transducer placed between two reflective gratings. This structure achieves low losses and handles sufficient power. It is used in most SAW duplexers. The SAW ladder filter however cannot be used in a balanced or differential mode.
Yet further, and with regard to devices known in the art, problems result with crosstalk due to crossover capacitance between an output and lateral transducer connections. This crosstalk results in filter rejection degradation and in degradation of the symmetry. In spite of the extensive efforts made in the art, there remains a need to provide a filter with desirable rejection (attenuation outside a passband) and symmetry while providing low output impedance.